In many types of vehicle seat belt retractors the design of the winding spring involves reaching a compromise between, on the one hand, providing a winding force large enough to wind the belt into the reel and, on the other hand, providing a winding force that is small enough to prevent causing discomfort to the occupant when the belt is in place. Because even a compromise in the design of the winding spring is not always satisfactory, usually because even a low winding force pulling the belt against the user's torso and waist can be bothersome, it is known to provide tension-relief mechanisms in seat belt retractors. The tension-relief mechanism may either lower the belt tension or eliminate the tension entirely when the belt is in place for proper restraint. Several known tension-relief mechanisms operate in response to a sequence of winding and unwinding motions of the belt.
Many of the tension-relief mechanisms proposed heretofore are relatively complicated, thus making the retractor more costly, often making it significantly larger in size, and sometimes increasing the chance of malfunction.